Principle of MCD spectroscopy

The sign of the derivative-shaped d term (which is equal to Aemax — Aamin) is positive when Aa is positive at high energy. 

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Principles of MCD Spectroscopy Applied to Hemes Optical and MCD Spectra of Ferrous and Ferric Hemes... 

Pure MCD with no rotation occurs if the sample is optically thick and completely absorbing in one circular polarisation, but not in the other. Pure MOR will occur when both circular polarisations are equally absorbed, i.e. the absorption coefficients ot+ v) and ot-(v) are equal, but the refractive indices n+(v) and n-(v) are not equal. The latter condition is satisfied at the centre of symmetry of the rotation pattern, viz. the field-free resonance frequency vq.6 In principle, the situation seems simpler when either pure MOR or pure MCD occurs, which is why most of the effort has traditionally been expended in separating one from the other, leading to MOR and MCD spectroscopies. 

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